I'm confused. XSD and RNG are to a large degree orthogonal - XSD is
a type system, RNG is an a schema language for XML. Perhaps XSD is
being conflated with WXS?

>While there are possible workarounds, I don't believe the current
>web-service tools or
>specs support using Relax NG.  And when explaining this to policy makers,
>a custom workaround without significant benefits won't fly.

Well less than two years ago, RNG was a fringe technology. Now it's
on the rise - I'm confident over the next two years RNG + son of XSD
will be the way we describe content models for web services. So I
suspect you'll see support for RNG in these tools. Although, if
policy makers are being diven by tools instead of technologies,
arguably they're putting the cart before the horse  - but that's a
different discussion.